Our most effective vaccines stimulate long-term neutralizing Ab responses by stimulating the immune system to maintain Ag-specific memory B cells and long-lived Ab-secreting plasma cells. In order to develop more effective Ab-stimulating vaccines in future, we need to improve our understanding of the mechanisms by which humoral immunity is induced and maintained. We reported that activation of CD1d- restricted NKT cells with the CD1d-binding glycolipid 1-Galactosylceramide (1-GC) at the time of immunization with a protein Ag resulted in enhanced Ab responses to that Ag. Since then we have observed that NKT cells enhance Ab recall responses following a secondary booster with Ag. We have also obtained data consistent with the hypothesis that the recall responses are a result of NKT cells enhancing memory B cell induction and plasma cell longevity. Despite this progress there are no current reports on the mechanisms by which NKT cells contribute to induction and maintenance of humoral immunity. Our preliminary data suggest that NKT cells utilize several mechanisms to induce and maintain an effective humoral immune response. These include (i) NKT-derived IL-5 which supports plasma cell induction (ii) NKT-expressed CD154 a ligand for CD40 expressed on B cells and perhaps important for the induction of primary Ab responses and memory B cells (iii) The TNF family member BAFF (B cell-activating factor) that promotes development and survival of plasma cells. In this proposal we will examine the mechanisms by which NKT cells impact humoral immune responses. Hypothesis and Aims: We will test the hypothesis that NKT cell-derived cytokines, CD154 and plasma cell survival factor BAFF contribute substantially to humoral immune responses by supporting the induction and maintenance of the memory B cell and long-lived plasma cell pool. In Specific Aim 1, we will assess the effects of CD1d-dependent NKT activation and NKT absence on humoral immune responses. In Specific Aim 2, we will determine how NKT-derived IL-5 and CD154 enhance humoral immune responses. In Specific Aim 3, we will determine if NKT-derived BAFF influences the induction of long-lived Ag-specific plasma cells. We will elucidate the mechanisms by which NKT cells support and enhance the induction and maintenance of humoral immune responses, thus integrating two previously diverse fields of research and substantially contributing to both. Our findings will be valuable for understanding humoral immunity and may contribute to the design of novel vaccination strategies that incorporate NKT activation.